Road luminaires are designed such that a certain luminance from the road is achieved with a required uniformity according to governmental specifications.
These specifications are particularly demanding in respect of the uniformity of the luminance in the direction of the road that the driver encounters in a particular lane. Moreover, it is required that the intensity of the light that can shine directly into a driver's eyes is limited. Too much light that shines directly into the driver's eyes leads to glare which can be dangerous for a driver. Thus, there is a careful balance in the light distribution for a road luminaire in the direction of the road that achieves the required uniformity and keeps the glare within the required specifications.
The preferred light source now used in road luminaires is a light emitting diode (LED) (in practice an array thereof), which typically emits the light in a Lambertian distribution. This distribution differs somewhat from the required light distribution.
A lens can be designed that is placed directly onto the LED to generate the required light distribution. An alternative to a luminaire consisting of LED plus lens is to use a tapered reflector placed around the LED plus an optical plate in front of the reflector to redirect the light to the required light distribution.
The optical plate may consist of micrometer to millimeter sized prismatic elements placed in a pixelated way.
The design and manufacture of the optical plate can however be complicated.
EP2690355A1 and US20090097248 both disclose a luminaire comprising a light source, a reflector arrangement and an optical plate with a prismatic structure with prismatic ridges extending in a side-to-side direction.